Magnetic Stimulation Coil Research Articles

Suppression of Heat Generation in Magnetic Stimulation Coil Applied for Treating Dysphagia

Suppression of Heat Generation in Magnetic Stimulation Coil Applied for Treating Dysphagia

"ConnectToBrain" : Synergy project for therapeutic closed-loop stimulation of brain network disorders

Therapeutic non-invasive transcranial brain stimulation with previous treatment protocols showed at best moderate effect sizes and large interindividual variability with asubstantial proportion of non-responders. A currently intensively discussed approach to address these problems is individualized closed-loop stimulation. ConnectToBrain is asynergy project funded by the European Research Council to develop noninvasive closed-loop therapeutic stimulation of network disorders of the human brain.It consists of three main pillars: (1)development of amultichannel transcranial magnetic stimulation (mTMS) coil array that covers nearly all of the cerebral cortex and enables highly precise electronic control of location, direction, intensity and timing of the induced electrical fields, (2)development of real-time analysis of activity and connectivity in brain networks using electroencephalography (EEG) for instantaneous spatial and temporal control of stimulation (brain state-dependent, closed-loop stimulation) and adaptive optimization of treatment effects by machine learning and (3)translation of these neurotechnological innovations into physiological and clinical studies.

Detection of axillary nerve multisegment motor nerve conduction using magnetic stimulation

Objective: To establish a method for detecting multi-segmental motor nerve conduction of axillary nerve using magnetic stimulation technique, and to preliminarily discuss its clinical application value. Methods: From April 2017 to February 2018, 80 healthy volunteers, 40 male and 40 female, were included and divided into five groups according to age and gender (groupⅠwere <35 years old; groupⅡwere 35-44 years old; group Ⅲ were 45-54 years old; groups Ⅳ were 55-64 years old; groups Ⅴ aged 65-74). Axillary nerve recording electrodes were placed in the deltoid muscle belly, reference electrode was placed in the deltoid muscle check point. Using the magnetic stimulation coil, stimulation was released at spots which were, respectively, 2 fingers from the lateral side of subscapular fossa, 2 fingers from the lateral side of infraclavicular fossae, and Erb's point. The latency, amplitude, detection distance (stimulation to recording electrode) were recorded at each spot, and the height and arm length were also measured. Results: The detection rate of axillary nerve waveform in healthy volunteers was 97.2%. There were no statistically significant differences in latency and amplitude in the different sides, gender (P>0.05).The latency corresponding to different detection distances was compared, and the difference was statistically significant (P<0.05).Height and arm length in different age groups were significantly increased in groupsⅠ, Ⅱ and Ⅲ compared with groups Ⅳ and Ⅴ, with statistical significance (P<0.05). When compared among different age groups, the latency and amplitude of group Ⅳand Ⅴ (more than 55 years old and over) were significantly prolonged and decreased, with statistically significant differences (P<0.05). Conclusions: The method of detecting multi-segmental magnetic stimulation of axillary nerve is stable and the result is reliable, which makes the evaluation of the proximal nerve of upper limbs be possible. However, the biological effects of age on nerve conduction should be carefully considered.

Electromagnetic and mechanical characterization of a flexible coil for transcranial magnetic stimulation

Transcranial magnetic stimulation is a painless and noninvasive method for treating brain disorders. The coil-geometries that match the head topologies achieve more effective stimulation, however it is difficult for a rigid coil to fit the skulls of all patients. We propose and develop a rubber-like flexible coil that can be shaped into different geometries to reduce the inter-individual variabilities in its clinical use. The main challenge is attributed to the fact that the external bending and induced Lorentzian forces cause coil deformation and fatigue. Herein, we investigated the influence of bending on the electromagnetic characteristics of the flexible coil. The magnetic field distribution was calculated and measured using a search coil. The induced Lorentzian force was calculated and the induced eddy current density was simulated using the scalar potential finite difference (SPFD) method. For a mechanical characterization, we fixed the center of the coil, and external bending forces were applied on the two wings of the coils, while Lorentzian forces were applied in a direction normal to the side wall of the wire groove. Fatigue analyses of these forces were also conducted. The results show that the eddy current density induced in the brain by the flexible coil was significantly higher compared to that of the figure-eight and butterfly coils. Fatigue analyses show that the bending force required to achieve a close coil fit on the human head and the generated Lorentzian force would not lead to fatigue problem.

Accessory to dissipate heat from transcranial magnetic stimulation coils

Background: Transcranial magnetic stimulation (TMS) produces magnetic pulses by passing a strong electrical current through coils of wire. Repeated stimulation accumulates heat, which places practical constraints on experimental design.New method: We designed a condensation-free pre-chilled heat sink to extend the operational duration of transcranial magnetic stimulation coils.Results: The application of a pre-chilled heat sink reduced the rate of heating across all tests and extended the duration of stimulation before coil overheating, particularly in conditions where heat management was problematic.Comparison with existing method: Applying an external heat sink had the practical effect of extending the operational time of TMS coils by 5.8–19.3 minutes compared to standard operating procedures.Conclusion: Applying an external heat sink increases the quantity of data that can be collected within a single experimental session.

Electrically Small Dipole Antenna Probe for Quasistatic Electric Field Measurements in Transcranial Magnetic Stimulation.

The present paper designs, constructs, and tests an electrically small dipole antenna probe for the measurement of electric field distributions with the ultimate purpose to directly measure electric fields induced by a transcranial magnetic stimulation (TMS) coil. Its unique features include applicability to measurements in both air and conducting medium, high spatial resolution, large frequency band from 100 Hz to 300 KHz, efficient feedline isolation via a printed Dyson balun, and accurate mitigation of noise. Prior work in this area is thoroughly reviewed. The proposed probe design is realized in hardware; implementation details and design tradeoffs are described. Test data are presented for the measurement of a constant wave capacitor electric field, demonstrating the probe's ability to properly measure electric fields caused by a charge distribution. Test data are also presented for the measurement of a constant wave solenoidal electric field, demonstrating the probe's ability to measure electric fields caused by Faraday's law of induction. Those are the primary fields for the transcranial magnetic stimulation. Further steps necessary for the application of this probe as an instrument for TMS coil design are discussed.

Development of Focused Transcranial Magnetic Stimulation Coils for Treating Schizophrenia

Transcranial magnetic stimulation (TMS) has the potential to treat a variety of psychiatric and neurological disorders. TMS has been used with success to treat major depressive disorder; however, researchers have been exploring the TMS therapy for many other neurological disorders including schizophrenia. Unfortunately, because the disorder is not well understood and exhibits an extremely wide range of symptoms, a successful therapy has proven to be challenging. Although the symptoms are diverse, auditory hallucinations are one of the most common positive symptoms and show the most promise for treatment using TMS. The primary auditory cortex (PAC) is thought to be one region of the brain mediating such hallucinations. Furthermore, this region is relatively small, which makes it difficult to isolate from surrounding cortical regions. In order to attempt to stimulate this area alone without stimulating the surrounding areas, a coil that provides more focused stimulation is desirable. The commercial “figure-of-eight coil” (FOE) is very popular for TMS research; however, another coil, the quadruple butterfly coil (QBC), has proven to provide more focal stimulation than the FOE. To further investigate this, we have performed finite-element simulations with both the FOE and the novel QBC on heterogeneous head models derived from magnetic resonance images of both healthy and schizophrenic individuals. Our results show that when the QBC is above the PAC, it stimulates more targeted cortical matter than the FOE, while still retaining the depth necessary to stimulate the target volume sufficiently. Furthermore, this result remained consistent across the healthy and schizophrenic models. These results support the use of the QBC coil to provide focused stimulation beyond that provided by the FOE.

Numerical Investigation of the Magnetic and Electric Field Distributions Produced by Biconical Transcranial Magnetic Stimulation Coil for Optimal Design

Transcranial magnetic stimulation (TMS) is a powerful tool for investigating the physiology of the human brain. Recently, the application of TMS in brain therapy has also received great interest. In this paper, a finite-element method is employed to investigate the distributions of magnetic and electric fields induced by the biconical stimulation coil in the two-shell spherical human head model. The structure and electrical parameters are optimized to achieve deep and accurate stimulation. The simulation results indicate that high stimulation current frequency can produce great electric field strength, with which a deep position in the brain can be stimulated. When the distance between two cones increases, the magnetic field distribution in the tissue fluid becomes less focusing and the stimulation depth decreases. In addition, reducing the intersection angle between the two parts of the biconical coil can lead to deeper and more focused stimulation. These results can be used for guiding the TMS design.

The strength and spread of the electric field induced by transcranial rotating permanent magnet stimulation in comparison with conventional transcranial magnetic stimulation

BackgroundWeak or low intensity transcranial stimulation of the brain, such as low field magnetic stimulation and electrical stimulation, can produce significant functional and therapeutic neuromodulatory effects. New methodWe have recently developed a portable wearable multifocal brain stimulator called transcranial rotating permanent magnet stimulator (TRPMS) that uses rapidly spinning high field strength permanent magnets attached to a cap. It produces oscillatory stimuli of different frequencies and patterns. Here we compared the strengths and spatial profiles of the changing magnetic fields of a figure-of-eight transcranial magnetic stimulator (TMS) coil, a TRPMS prototype, and a scaled-up version of TRPMS. We measured field strengths and directions of voltages induced in a magnetic field sensor oriented along all three orthogonal axes. Results and comparison with existing methodsThe spatial spread of the TRPMS-induced electric field is more restricted, and its shape and strength vary less with the orientation of the inductance than TMS. The maximum voltage induced by the current prototype is ∼7% of the maximal TMS output at depths corresponding to the human cerebral cortex from the scalp surface. This field strength can be scaled up by a factor ∼8 with a larger diametrically magnetized magnet. These comparative data allow us to estimate that intracortical effects of TRPMS could be stronger than other low intensity stimulation methods. ConclusionsTRPMS might enable greater uniformity, consistency and focality in stimulation of targeted cortical areas subject to significant anatomical variability. Multiple TRPMS microstimulators can also be combined to produce patterned multifocal spatiotemporal stimulation.

Development and characterization of the InVesalius Navigator software for navigated transcranial magnetic stimulation

BackgroundNeuronavigation provides visual guidance of an instrument during procedures of neurological interventions, and has been shown to be a valuable tool for accurately positioning transcranial magnetic stimulation (TMS) coils relative to an individual’s anatomy. Despite the importance of neuronavigation, its high cost, low portability, and low availability of magnetic resonance imaging facilities limit its insertion in research and clinical environments. New methodWe have developed and validated the InVesalius Navigator as the first free, open-source software for image-guided navigated TMS, compatible with multiple tracking devices. A point-based, co-registration algorithm and a guiding interface were designed for tracking any instrument (e.g. TMS coils) relative to an individual’s anatomy. ResultsLocalization, precision errors, and repeatability were measured for two tracking devices during navigation in a phantom and in a simulated TMS study. Errors were measured in two commercial navigated TMS systems for comparison. Localization error was about 1.5 mm, and repeatability was about 1 mm for translation and 1° for rotation angles, both within limits established in the literature. Comparison with existing methodsExisting TMS neuronavigation software programs are not compatible with multiple tracking devices, and do not provide an easy to implement platform for custom tools. Moreover, commercial alternatives are expensive with limited portability. ConclusionsInVesalius Navigator might contribute to improving spatial accuracy and the reliability of techniques for brain interventions by means of an intuitive graphical interface. Furthermore, the software can be easily integrated into existing neuroimaging tools, and customized for novel applications such as multi-locus and/or controllable-pulse TMS.

Associative plasticity in supplementary motor area - motor cortex pathways in Tourette syndrome

The important role of the supplementary motor area (SMA) in the generation of tics and urges in Gilles de la Tourette syndrome (GTS) is underscored by an increased SMA-motor cortex (M1) connectivity. However, whether plasticity is also altered in SMA-M1 pathways is unclear. We explored whether SMA-M1 plasticity is altered in patients with Tourette syndrome. 15 patients with GTS (mean age of 33.4 years, SD = 9.9) and 19 age and sex matched healthy controls were investigated with a paired association stimulation (PAS) protocol using three transcranial magnetic stimulation (TMS) coils stimulating both M1 and the SMA. Standard clinical measures for GTS symptoms were collected. There was a significant PAS effect showing that MEP amplitudes measured in blocks during and after PAS were significantly higher compared to those in the first block. However, the degree of PAS was not differentially modulated between patients and controls as shown by a Bayesian data analysis. PAS effects in GTS correlated positively with the YGTSS motor tic severity. Plasticity previously reported to be altered in sensorimotor pathways in GTS is normal in SMA-M1 projections suggesting that the dysfunction of the SMA in GTS is not primarily related to altered plasticity in SMA-M1 connections.

Electric field measurement of two commercial active/sham coils for transcranial magnetic stimulation

Objective. Sham TMS coils isolate the ancillary effects of their active counterparts but typically induce low-strength electric fields (E-fields) in the brain, which could be biologically active. We measured the E-fields induced by two pairs of commonly-used commercial active/sham coils. Approach. E-field distributions of the active and sham configurations of the Magstim 70 mm AFC and MagVenture Cool-B65 A/P coils were measured over a 7 cm-radius, hemispherical grid approximating the cortical surface. Peak E-field strength was recorded over a range of pulse amplitudes. Main results. The Magstim and MagVenture shams induce peak E-fields corresponding to 25.3% and 7.72% of their respective active values. The MagVenture sham has an E-field distribution shaped like its active counterpart. The Magstim sham induces nearly zero E-field under the coil’s center, and its peak E-field forms a diffuse oval 3–7 cm from the center. Electrical scalp stimulation paired with the MagVenture sham is estimated to increase the sham E-field in the brain up to 10%. Significance. Different commercial shams induce different E-field strengths and distributions in the brain, which should be considered in interpreting outcomes of sham stimulation.

Development of peripheral magnetic stimulation system to stimulate suprahyoid muscles

Neuromuscular electrical stimulation has been widely used in patients with dysphagia, but obtaining sufficient hyoid bone movement through surface electrodes seems difficult due to pain. On the other hand, magnetic stimulation activates the nerve and the muscles without stimulating the skin nociceptors. However, the coil for magnetic stimulation is too big to stimulate smaller muscles like the suprahyoid muscles. The aim of this study was to develop the peripheral magnetic stimulation system to stimulate suprahyoid muscles. This study was approved by the institutional review board. We designed a smaller coil to stimulate suprahyoid muscles without stimulating inferior alveolar nerve that induced pain to the teeth. We also developed a jig to support the coil. Eleven healthy subjects aged 32 ± 9 years participated in this study. The new coil was placed on the submental area of the subjects and the magnetic stimulation was performed one time at a rate of 30 Hz for 2 s. The intensity level selected induces hyoid bone movement without causing intolerable pain to the subjects. The hyoid bone at rest between on- and off-magnetic stimulations of the suprahyoid muscles were identified using fluoroscopy at 30 frames/s in lateral projection and the displacements of the hyoid bone were calculated using Image J program. Pain during peripheral magnetic stimulation was assessed from 0 to 10 using the numerical rating scale (NRS). Magnetic stimulation resulted in 10.9 ± 2.8 mm forward displacement and 8.3 ± 4.1 mm upward displacement of the hyoid bone. The median NRS score during magnetic stimulation was 1. We obtained similar extent of hyoid bone movement at rest compared with normal drinking of 10-mL liquid. The peripheral magnetic stimulation can become an alternative to electrical stimulation for patients with reduced laryngeal elevation.

Simulation Study on Coil Design for Transcranial Magnetic Stimulation.

The purpose of this study is to optimize the coil for transcranial magnetic stimulation (TMS). A novel multi-circle tangent coil (MTC) was designed, which consisted of five inner circular coils and one outer circular coil. These six circular coils were tangent to each other. The induced electric field and target area of MTC were investigated with both homogeneous phantom model and realistic human head model by numerical simulation. The results showed that MTC induced low electric field along its 4 surrounding inner coils thus weakened the stimulus effect on non-target areas. MTC not only takes the advantages of figure-8 coils with high focality, but also enables deep stimulation and multi-point stimulation at the same time. MTC is expected to have great potential in clinical application.

Design of transcranial magnetic stimulation coils with optimal trade-off between depth, focality, and energy

Objective. Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique used for research and clinical applications. Existent TMS coils are limited in their precision of spatial targeting (focality), especially for deeper targets. This paper presents a methodology for designing TMS coils to achieve optimal trade-off between the depth and focality of the induced electric field (E-field), as well as the energy required by the coil. Approach. A multi-objective optimization technique is used for computationally designing TMS coils that achieve optimal trade-offs between E-field focality, depth, and energy (fdTMS coils). The fdTMS coil winding(s) maximize focality (minimize the volume of the brain region with E-field above a given threshold) while reaching a target at a specified depth and not exceeding predefined peak E-field strength and required coil energy. Spherical and MRI-derived head models are used to compute the fundamental depth–focality trade-off as well as focality–energy trade-offs for specific target depths. Main results. Across stimulation target depths of 1.0–3.4 cm from the brain surface, the suprathreshold volume can be theoretically decreased by 42%–55% compared to existing TMS coil designs. The suprathreshold volume of a figure-8 coil can be decreased by 36%, 44%, or 46%, for matched, doubled, or quadrupled energy. For matched focality and energy, the depth of a figure-8 coil can be increased by 22%. Significance. Computational design of TMS coils could enable more selective targeting of the induced E-field. The presented results appear to be the first significant advancement in the depth–focality trade-off of TMS coils since the introduction of the figure-8 coil three decades ago, and likely represent the fundamental physical limit.

The challenges of producing effective small coils for transcranial magnetic stimulation of mice

Introduction. Transcranial magnetic stimulation (TMS) is used for treating neurological disorders. Rapid pulses of magnetic field are delivered via a high-current coil situated over the scalp and induce an electric field in the brain. There has been limited fundamental scientific research on TMS and to progress it would be ideal to mimic the electric field of human TMS with mice. Animal models provide good mechanistic insight, but their use is hindered by lack of stimulating coils comparable in focus and intensity with human stimulation. Methods. We outline the engineering challenges in producing appropriate coils. It is unclear what should be optimized in the design of a mouse coil. We model the electric field, heat generation and ring-down time for cylindrical coils and use results to select a coil design consisting of 70 turns of 0.4 mm diameter copper wire wrapped around a 5 mm diameter soft ferrite core. Results and Discussion. While the magnetic flux density scales as the reciprocal of length-scale, the electric field does not scale with length, meaning that a large current is required to mimic the electric field of humans. To maximize electric field, one must minimize the coil’s inductance resulting in reduced ring-down time for the coil and significant heating. A ferrite core allows ring-down time to remain high and reduces heating. Our coil gave 180 mT at 30 V supply, with a temperature increase of 5°C after 1200 pulses at 5 Hz. The B-field below the core has a full-width-at-half-maximum of 6 mm, similar in size to a mouse brain. Conclusions. We have produced a mouse coil that offers increased B-field and reduced heating. There is considerable scope for improving electric field, but further physical analysis may lead to field strength more similar to that obtained in human TMS.

Redesigning existing transcranial magnetic stimulation coils to reduce energy: application to low field magnetic stimulation

Objective. To present a systematic framework and exemplar for the development of a compact and energy-efficient coil that replicates the electric field (E-field) distribution induced by an existing transcranial magnetic stimulation coil. Approach. The E-field generated by a conventional low field magnetic stimulation (LFMS) coil was measured for a spherical head model and simulated in both spherical and realistic head models. Then, using a spherical head model and spatial harmonic decomposition, a spherical-shaped cap coil was synthesized such that its windings conformed to a spherical surface and replicated the E-field on the cortical surface while requiring less energy. A prototype coil was built and electrically characterized. The effect of constraining the windings to the upper half of the head was also explored via an alternative coil design. Main results. The LFMS E-field distribution resembled that of a large double-cone coil, with a peak field strength around 350 mV m−1 in the cortex. The E-field distributions of the cap coil designs were validated against the original coil, with mean errors of 1%–3%. The cap coil required as little as 2% of the original coil energy and was significantly smaller in size. Significance. The redesigned LFMS coil is substantially smaller and more energy-efficient than the original, improving cost, power consumption, and portability. These improvements could facilitate deployment of LFMS in the clinic and potentially at home. This coil redesign approach can also be applied to other magnetic stimulation paradigms. Finally, the anatomically-accurate E-field simulation of LFMS can be used to interpret clinical LFMS data.

Comparative Study of a New Coil Design With Traditional Shielded Figure-of-Eight Coil for Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a non-invasive neuro-stimulation based on the principle of electromagnetic induction that uses brief, strong magnetic pulses of electric current delivered to a coil placed on the subject’s head to induce an electric field in the brain. This electric field could stimulate and modulate neural activity. Induced E-field inside brain generated by repetitive TMS (rTMS) can produce changes in neural activity that extends beyond the period of stimulation. Therefore, rTMS can be used as a probe for exploring higher brain functions and as potential treatment technique for psychiatric and neurological disorders. However, based on the chosen shape of the excitation coil, there exists a tradeoff between the excitation focality in the brain and the depth to which the field penetrates. In this paper, a new design of TMS coil is proposed to enhance the performance of the traditional figure-of-eight (FOE) coil. 3-D finite-element method has been used to simulate and compare the electric field induced inside the model of a human brain under the influence of both coil designs. An experimental verification was then followed to validate the advantages of the proposed coil design over the classic FOE coil.

Trial Manufacture of Magnetic Stimulation Coil to Induce the Contraction of Suprahyoid Muscles

Trial Manufacture of Magnetic Stimulation Coil to Induce the Contraction of Suprahyoid Muscles

Improved Figure-of-Eight Coil for Transcranial Magnetic Stimulation Using Magnetic Resonant Coupling

This paper proposes an improved figure-of-eight coil for transcranial magnetic stimulation (TMS) by using the magnetic resonant coupling (MRC) effect, aiming to improve the treatment effect for psychiatric disorders. The proposed MRC-based figure-of-eight coil consists of two reverse-winding resonance coils, which embraces two series-connected source coils, respectively. Different from conventional figure-of-eight coils, the proposed topology can produce the resonant electromagnetic field to accumulate the stimulation energy. In such ways, the improved TMS system can effectively enhance the stimulation focality and depth while significantly reducing the requirement of the power supply, thus improving the safety as well as facilitating the miniaturization and portability. Last, this paper gives the simulated and experimental results to verify the theoretical analysis, which can further illustrate the feasibility of the proposed MRC-based figure-of-eight coil for TMS systems.